The present invention relates to polymer filaments which have a high tensile strength and a high modulus, and to a process for making such filaments.
Filaments are usually made by spinning linear polymers. A polymer is first made into a liquid such as a melt or a solution and is then spun forming a filament. Although other substances are capable of being spun the chain formation of the macromolecules is an important consideration as side branches have an adverse effect on filament formation and mechanical properties. Therefore, the production of filaments in accordance with the process of the present invention is premised on the use of linear polymers although a limited degree of branching is usually unavoidable and will have to be accepted.
The randomly oriented chains of molecules in this filament must next be oriented lengthwise in the filament which is accomplished by stretching.
Stretching of the filament results in orienting the chain macromolecules lengthwise which also increases the strength of the filaments. However, in many cases the strength of the stretched filaments is still far below the value that is theoretically expected. Many attempts have already been made at producing filaments with a tensile strength and a modulus closer to what are theoretically possible. These attempts, of which a survey is given in publications by Juyn in Plastica 31 (1978) 262-270 and Bigg in Polymer Eng. Sci. 16 (1976) 725-734, have failed to yield satisfactory results. In a number of cases the modulus ws improved sufficiently, but not the tensile strength. There was an additional drawback in that the filament formation was so slow that economic production would be impossible.
It has now been found that polymer filaments which have a high tensile strength and a high modulus can be made by stretching a polymer filament containing an appreciable amount of polymer solvent at a temperature between the swelling point and the melting point of the polymer. Preferably a spinnable solution is spun by any of the known methods, the resultant filament is cooled to below the solution temperature of the polymer, the temperature of the filament is then adjusted at a value between the swelling point of the polymer in the solvent and the melting point of the polymer, and the filament is then stretched.
In the dry spinning process which is widely applied on a technical scale, a solution of a spinnable polymer is spun in a shaft through which air is blown to evaporate all or mosts of all of the solvent to form the filament. While the air is usually heated the temperature in the shaft is kept below the melting point of the polymer in order to increase the mechanical strength of the filament, which is very low as it exits the spinning aperture.
The mechanical strength can be further increased in the subsequent stretching step at temperatures below the melting point of the polymer.